Sure, Peter's arguably being kind of a hard-ass here, but at least he's not a bullshitter. No smiling and fake gladness at encountering someone with whom he has a fraught history, then cursing him out to their old friends behind his back. Sometimes there's just nothing more to say- one of those times is when a guy who was on a path to destruction and was doomed in the eyes of everyone he knew manages to break free (good for him) but without a care leaves old acquaintances in an emotional limbo. And yes, 'it's Patrick's life'- Peter's thinking exactly that, and he's leaving him to go live it. There's nothing more he can say.
Peter needs some growing up to do. Getting older and leaving friends behind is absolutely normal. You change. The others change. You move cities, you change jobs, you get married and have kids and have no time anymore. Life is like that.
From page one Peter has this clearly pathologic inability to move on. Everybody has found their own thing now and they're clearly doing better, so why does he pine for the days when they did nothing but drink and smoke pot all day? That's not growth.